Actually, typeglobs are more like structures than unions. Symbol table entries have a name field plus a value field for each type of value.

As for why the Perl symbol table was designed this way, my guess is that it was just easier to manage only one entry per name and, similarly, only one symbol table per package.

Personally, I think it would have been simpler to only allow a given symbol to be one type of value, that of its first use. Since this is in line with best coding practices, it would have been a reasonable design choice.